This invention relates to a device which is adapted to be positioned in the path of a beam of electromagnetic radiation propagating in free space which changes characteristics of the beam. The invention is particularly, but not exclusively, concerned with microwave devices.
The term microwave refers to the part of the electromagnetic spectrum substantially in the frequency range 0.2 to 300 GHz. It includes that part of the spectrum referred to as millimeter wave (having a frequency in the range 30 to 300 GHz).
In a known device for controlling the direction of a microwave beam, the microwave beam passes through a rectangular block of dielectric material formed by two wedge-shaped pieces, one being of ferrite material and one being of non-ferrite material, the pieces having their sloping faces in juxtaposition. An external magnetic field is applied to the block in a direction perpendicular to the direction of propagation of the microwave beam. The magnetic field is substantially constant across the block.
Applied magnetic field induces magnetization in the material which is substantially uniform across the block. A microwave beam passing through the magnetised material will interact with it and this interaction changes relative velocity across the beam. If a microwave beam is directed through the block so as to travel in turn through a thickness of the ferrite and then through a thickness of the non-ferrite material, certain parts of the beam will travel through a different length of ferrite material compared to certain other parts of the beam thus causing a differential phase shift across the block. The phase at one edge will lag when compared to the phase at the other edge and the beam will be deflected. Altering the direction of the magnetic field will cause the beam to deflect in an opposite direction.
In another embodiment of a device for controlling the direction of a microwave beam, the beam passes through a cylinder of material formed by two wedge-shaped pieces one being of ferrite and one being of non-ferrite material, the pieces having the sloping faces in juxtaposition. The cylinder is located within an external solenoid which is used to apply a magnetic field along the longitudinal axis of the cylinder which is substantially parallel to the direction of propagation of the beam. The magnetic field is substantially constant across the cylinder. The device operates by Faraday rotation. For circularly polarized beams such a device induces a differential phase shift in the beam thus causing deflection of the beam. Linearly polarized beams are equivalent to a combination of two circularly polarized beams rotating in opposite directions and so such a device splits a linearly polarized beam into two separate circularly polarized beams leaving the device at angles +.theta..degree. and -.theta..degree. to the direction of propagation of the original beam.
Devices of this kind are difficult to construct and cause in-line loss due to beam reflection at the junction between the ferrite and non-ferrite wedge shaped pieces. Such devices provide beam deflection in one plane only and so two devices in series would be required to produce conical steering.
Another device for controlling the direction of a microwave beam comprises a body of ferrite material having magnetic coils which apply a magnetic field across the body which induces a gradient in magnetization across the body. The resultant direction of the beam leaving the device is perpendicular to the gradient in the magnetic field across the body. Therefore the degree of deflection in the beam is controlled by the gradient in the magnetization. The device differs from the two devices described above in that all parts across the width of a microwave beam pass through the same thickness of ferrite material. However magnetization induced varies across the ferrite material through which the microwave beam passes.
A disadvantage with this device is that the thickness of the body is governed by its width. If the body is relatively thin compared to its width, magnetic flux tends to concentrate around the coils and so does not penetrate sufficiently across the width of an aperture through which the beam passes and little or no magnetic flux passes through the body in a central region of the aperture. However, the width of the material is governed by the width of the beam which the device is to steer and so cannot be chosen independently. As a result devices of this type need to have a thickness and a width which are comparable. This causes the devices to be bulky, heavy, cumbersome and expensive. Furthermore a thicker material causes greater insertion loss in a system.